Lift systems for raising oil from oil wells are often mechanical systems that pressurize the fluid within the well so that the well fluids can be raised to the surface. Referring to FIG. 1, a conventional lift system for an oil well 10 is shown. As shown, a wellbore 12 of the oil well 10 penetrates a subterranean formation 14 to a producing zone 16 of the formation where oil is located. The producing zone 16 may be a zone that has been hydraulically fractured using known fracturing techniques to facilitate oil production.
A casing 18 may be provided to line the wellbore 12. The casing 18 and/or wellbore 12 may be perforated in the producing zone 16 with perforations 20 that allow the flow of well fluids from the producing zone 16 into the wellbore 12 and casing 18.
A tubing string 22 is provided that extends from a wellhead 24 located at the surface and downward through the interior of the casing 18. An inlet 26 of the tubing string 22 extends far down into the well at or adjacent to the lowermost perforations 20 to allow well fluids that enter the perforated casing 18 to enter the lower end of the tubing string 22.
A reciprocating pump assembly 28 (e.g., barrel pump) is provided with the tubing string 22 to facilitate pumping of well fluids upward through the tubing of the tubing string to the wellhead 24. The pump assembly 28 is operated by means of a pump jack 30 located at the surface that is coupled to the pump 28 through a sucker rod 32 that extends from the pump jack 30 through the tubing string 22 to the pump assembly 28 for actuating the reciprocating pump 28.
An annulus 34 surrounds the tubing string 22 and is defined by the space between the exterior of the tubing string 22 and the interior of the well casing 18. Gas 36 may flow through the annulus to the surface where it may be collected at the wellhead 24 and discharged through gas outlet 38.
Oil 40 from the formation is pumped by operation of the reciprocating pump 28 wherein oil and other liquids enter through inlet 26 and are passed upward through the tubing string 22 to the wellhead 24 and discharged through outlet 42. Oftentimes, a gas/liquid separator (not shown) is provided with the tubing string 22 so that gases from the formation are discharged into the annulus 34 before entering the pump 28.
Because the lift system of FIG. 1 requires the use of a mechanical downhole pump assembly 28, the pump assembly is subject to the extreme conditions of the downhole environment. This often leads to pump failure. Sand and grit from the formation that is entrained in the oil tends to wear and damage the components of the pump so that the pump must be routinely removed and repaired or replaced, thus negatively impacting production.
Lift systems that employ downhole pumps, such as those of FIG. 1, also become less efficient when the amount of oil flow from the formation is low. In stripper wells, i.e., wells that produce less than 10 barrels of oil per day, other means are often used to lift the oil from the well. One of these is the gas lift system.
FIG. 2 shows the use of a conventional gas lift system with an oil well 43. The components of the oil well 43 are similar to those of the oil well 10 of FIG. 1, with similar components being labeled with the same reference numerals. In the oil well 43, gas is injected at high pressure into the annulus 34 from a gas inlet 44 at the surface. The tubing string 22 is provided with a series of gas valves 46 located along its length. As injected gas flows downward through the annulus 34 it flows into the uppermost valve 46 so that the gas enters the tubing 22 through the valve 46. This gas mixes with the fluids within the tubing 22 so that the density of the fluids is decreased and they are lifted through the tubing 22 to the wellhead 24 and the liquid/gas mixture is discharged through outlet 48.
As the liquid level within the annulus 34 drops below the uppermost valve 46, it will close and the gas will be introduced into the valve 46 beneath it. As the liquid level continues to drop, each successive valve 46 is closed so that gas from the annulus 34 may enter into the valve where both liquid and gas flow upward through the tubing and out outlet 48.
Like the downhole pump assemblies of FIG. 1, the conventional gas lift system requires the tubing string to extend far into the wellbore, typically terminating at or near the perforations, so that the well fluids can continue to be withdrawn as the liquid level within the annulus drops. The gas lift system also requires the need for added equipment in the form of the mechanical gas lift valves that are provided along the length of the tubing string, thus adding to the complexity and cost of the lift system. Like the reciprocating pump assemblies, the gas valves having moving parts that may also be prone to failure, such as from wear from sand and grit, and may require periodic removal, repair or replacement, which negatively impacts production.
One of the shortcomings of the prior art systems is maintaining proper flow of oil from the formation. In formations that are hydraulically fractured, fractures that radiate from the wellbore into the producing zone(s) facilitate the flow of fluids from the formation. Hydrostatic head pressure from the well fluids in the wellbore help to keep these fractures open. In both the conventional pumping and gas lift systems, the tubing string inlet is located near the perforations and fractures. Continued lifting of fluids therefore causes the liquid levels in the wellbore to be drawn down to the point of where the hydrostatic head pressure that facilitates opening of the fractures is lost and the fractures close. While proppants can facilitate maintaining these fractures open to a certain degree, they are not as efficient opening the fractures as a high hydrostatic head pressure.
Accordingly, a need therefore exists to overcome the shortcomings of these prior art systems.